Centrifugal separators typically comprise a vessel with a cylindrical wall which is rotated about its axis. A mixture of fluids of different specific gravities is introduced and concentric annular layers of the individual fluids are formed, with the fluid of greatest specific gravity forming the outermost layer against the cylindrical wall and with the liquid with the least specific gravity forming the layer nearest the axis. The separation effected in this way within the centrifuge has of course to be maintained during extraction of the liquids from it, in spite of varying proportions of the liquid in the incoming mixtures. Operation of the centrifuge can be controlled by a flow control system dependent on the use of sensing devices to detect the positions of the level of the layers or radial interface between them, as described, for example in U.S. Pat. No. 4,846,780. The level or interface sensing means and related control arrangements represent a considerable complication, making a substantial contribution to the complexity and cost of the equipment.